Various forms of door security lock structures have been provided. Some of these security lock structures include features which enable a door to be partially opened, but locked against unrestricted further opening to enable a person on the inside of a door to view the area outside of the door. However, many of these security locks utilize chains for accomplishing the locking function of the door in a partially opened position and chains allow an intruder to initially pull the door toward a closed position and then to forcibly move the door toward an open position with momentum of the intruder and the door being sufficient, in most cases, to break the door restraining chain. Accordingly, a need exists for an improved form of security lock for doors which will enable doors to be securely locked in a partially opened position not only against further movement toward the open position but also against movement toward the closed position.
Further, U.S. Pat. No. 4,374,599 shows a security lock for door that has a first stationary flange supported from the free swinging edge of a door, and a second pivoted flange mounted on the door jamb and opposing the stationary flange. U.S. Pat. No. 7,107,800 discloses a door bolt which is constructed to include a jam plate fixed to a door jamb and a doorplate assembly installed in the free end of adoor panel and adapted to secure the free end of the door panel to the jamb plate. Further examples of various forms of door security locks including some of the general structural and operational features of the instant invention are disclosed in U.S. Pat. Nos. 267,035, 771,588, 2,461,398, 2,562,916 and 3,411,817.